Wide-band Imaging

by stephanw — last modified Dec 05, 2013 by Gustaaf Van Moorsel

The very wide bandpasses provided by the Jansky Very Large Array enable imaging over 2:1 bandwidth ratios -- at L, S, and C bands, the upper frequency is twice that of the lower frequency.   It is this wide bandwidth which enables sub-microJy sensitivity.

In many cases, where the observation goal is a simple detection, and there are no strong sources near to the region of interest, standard imaging methods that combine the data from all frequencies into one single image (multi-frequency-synthesis) may suffice.  This is because the wide-band system makes a much better synthesized beam -- especially for longer integrations -- than the old single-frequency beam, thus considerably reducing the region of sky which is affected by incorrect imaging/deconvolution.  A rough rule of thumb is that -- provided a strong source is not adjacent to the target zone -- if the necessary dynamic range in the image is less than 1000:1, (i.e., the strongest source in the beam is less than 1000 times higher than the noise), a simple wide-band map may suffice.  

For higher dynamic ranges, complications arise from the fact that the brightness in the field of view dramatically changes as a function of frequency, both due to differing structures in the actual sources in the field of view, and due to the attenuation of the sources by the primary beam.  One symptom of such problems is the appearance of radial spokes around bright sources, visible above the noise floor, when imaged as described above.  

The simplest solution is to simply make a number of maps (say, one for each subband), which can then be suitably combined after correction for the primary beam shape. But with up to 64 subbands available with the VLA's new correlator, this is not always the optimal approach.  Further, images at all bands must be smoothed to the angular resolution at the lowest frequency before any spectral information can be extracted, and with a 2:1 bandwidth the difference in angular resolution across the band will be significant.  

A better approach is to process all subbands simultaneously, utilizing software which takes into account the possibility of spatially variant spectral index and curvature, and knows the instrumentally-imposed attenuation due to the primary beam. Such wideband imaging algorithms are now available within CASA as part of the clean task, and work is under way to integrate them fully with wide-field imaging techniques.